The Pavement designs given in the previous edition IRC:37-1984 were applicable
to design traffic upto only 30 million standard axles (msa).
The earlier code is empirical in nature which has limitations regarding
applicability and extrapolation.
This guidelines follows analytical designs and developed new set of designs up
to 150 msa.
These guidelines will apply to design of flexible pavements for Expressway,
National Highways, State Highways, Major District Roads, and other categories
Flexible pavements are considered to include the pavements which have
bituminous surfacing and granular base and sub-base courses conforming to IRC/
MOST standards. These guidelines apply to new pavements.
The flexible pavements has been modeled as a three layer structure and stresses
and strains at critical locations have been computed using the linear elastic
To give proper consideration to the aspects of performance, the following three
types of pavement distress resulting from repeated (cyclic) application of
traffic loads are considered:
While the permanent deformation within the bituminous layer can be controlled
by meeting the mix design requirements, thickness of granular and bituminous
layers are selected using the analytical design approach so that strains at the
critical points are within the allowable limits.
For calculating tensile strains at the bottom of the bituminous layer, the
stiffness of dense bituminous macadam (DBM) layer with 60/70 bitumen has been
used in the analysis.
- vertical compressive strain at the top of the sub-grade which can cause
sub-grade deformation resulting in permanent deformation at the pavement
- horizontal tensile strain or stress at the bottom of the bituminous layer
which can cause fracture of the bituminous layer.
- pavement deformation within the bituminous layer.
A and B are the critical locations for tensile strains . Maximum
value of the strain is adopted for design.
C is the critical location for the vertical subgrade strain
since the maximum value of the occurs mostly at C.
Critical Locations in Pavement
Bituminous surfacing of pavements display flexural fatigue cracking if the
tensile strain at the bottom of the bituminous layer is beyond certain limit.
The relation between the fatigue life of the pavement and the tensile strain in
the bottom of the bituminous layer was obtained as
in which, is the allowable number of load repetitions to control fatigue
cracking and is the Elastic modulus of bituminous layer.
The use of equation would result in fatigue cracking of 20% of the
The allowable number of load repetitions to control permanent deformation can
be expressed as
is the number of cumulative standard axles to produce rutting of 20 mm.
Based on the performance of existing designs and using analytical approach,
simple design charts and a catalogue of pavement designs are added in the code.
The pavement designs are given for subgrade CBR values ranging from 2% to 10%
and design traffic ranging from 1 msa to 150 msa for an average annual pavement
temperature of 35 C.
The later thicknesses obtained from the analysis have been slightly modified to
adapt the designs to stage construction.
Using the following simple input parameters, appropriate designs could be
chosen for the given traffic and soil strength:
- Design traffic in terms of cumulative number of standard axles; and
- CBR value of subgrade.
The method considers traffic in terms of the cumulative number of standard
axles (8160 kg) to be carried by the pavement during the design life.
This requires the following information:
- Initial traffic in terms of CVPD
- Traffic growth rate during the design life
- Design life in number of years
- Vehicle damage factor (VDF)
- Distribution of commercial traffic over the carriage way.
Initial traffic is determined in terms of commercial vehicles per day (CVPD).
For the structural design of the pavement only commercial vehicles are
considered assuming laden weight of three tonnes or more and their axle loading
will be considered.
Estimate of the initial daily average traffic flow for any road should normally
be based on 7-day 24-hour classified traffic counts (ADT).
In case of new roads, traffic estimates can be made on the basis of potential
land use and traffic on existing routes in the area.
Traffic growth rate
Traffic growth rates can be estimated (i) by studying the past trends of
traffic growth, and (ii) by establishing econometric models.
If adequate data is not available, it is recommended that an average annual
growth rate of 7.5 percent may be adopted.
For the purpose of the pavement design, the design life is defined in terms of
the cumulative number of standard axles that can be carried before
strengthening of the pavement is necessary.
It is recommended that pavements for arterial roads like NH, SH should be
designed for a life of 15 years, EH and urban roads for 20 years and other
categories of roads for 10 to 15 years.
Vehicle Damage Factor
The vehicle damage factor (VDF) is a multiplier for converting the number of
commercial vehicles of different axle loads and axle configurations to the
number of standard axle-load repetitions.
It is defined as equivalent number of standard axles per commercial vehicle.
The VDF varies with the axle configuration, axle loading, terrain, type of
road, and from region to region.
The axle load equivalency factors are used to convert different axle load
repetitions into equivalent standard axle load repetitions. For these
equivalency factors refer IRC:37 2001.
The exact VDF values are arrived after extensive field surveys.
A realistic assessment of distribution of commercial traffic by direction and
by lane is necessary as it directly affects the total equivalent standard axle
load application used in the design.
Until reliable data is available, the following distribution may be assumed.
- Single lane roads: Traffic tends to be more channelized on single
roads than two lane roads and to allow for this concentration of wheel load
repetitions, the design should be based on total number of commercial vehicles
in both directions.
- Two-lane single carriageway roads: The design should be based on 75
% of the commercial vehicles in both directions.
- Four-lane single carriageway roads: The design should be based on
40 % of the total number of commercial vehicles in both directions.
- Dual carriageway roads: For the design of dual two-lane carriageway
roads should be based on 75 % of the number of commercial vehicles in each
For dual three-lane carriageway and dual four-lane carriageway the distribution
factor will be 60 % and 45 % respectively.
For the design of pavements to carry traffic in the range of 1 to 10 msa, use
chart 1 and for traffic in the range 10 to 150 msa, use chart 2 of IRC:37 2001.
The design curves relate pavement thickness to the cumulative number of
standard axles to be carried over the design life for different sub-grade CBR
values ranging from 2 % to 10 %.
The design charts will give the total thickness of the pavement for the above
The total thickness consists of granular sub-base, granular base and bituminous
The individual layers are designed based on the the recommendations given below
and the subsequent tables.
Sub-base materials comprise natural sand, gravel, laterite, brick metal,
crushed stone or combinations thereof meeting the prescribed grading and
The sub-base material should have a minimum CBR of 20 % and 30 % for traffic
upto 2 msa and traffic exceeding 2 msa respectively.
Sub-base usually consist of granular or WBM and the thickness should not be
less than 150 mm for design traffic less than 10 msa and 200 mm for design
traffic of 1:0 msa and above.
The recommended designs are for unbounded granular bases which comprise
conventional water bound macadam (WBM) or wet mix macadam (WMM) or equivalent
confirming to MOST specifications.
The materials should be of good quality with minimum thickness of 225 mm for
traffic up to 2 msa an 150 mm for traffic exceeding 2 msa.
The surfacing consists of a wearing course or a binder course plus wearing
The most commonly used wearing courses are surface dressing, open graded premix
carpet, mix seal surfacing, semi-dense bituminous concrete and bituminous
For binder course, MOST specifies, it is desirable to use bituminous macadam
(BM) for traffic upto o 5 msa and dense bituminous macadam (DBM) for traffic
more than 5 msa.
Design the pavement for construction of a new bypass with the following data:
- Two lane carriage way
- Initial traffic in the year of completion of construction = 400 CVPD (sum
of both directions)
- Traffic growth rate = 7.5 %
- Design life = 15 years
- Vehicle damage factor based on axle load survey = 2.5 standard axle per
- Design CBR of subgrade soil = 4%.
Basic concepts of flexible pavement design were discussed.
There are two main design procedures- empirical and mechanistic empirical
For design purposes, equivalent single wheel load and equivalent single axle
load concepts are used.
- Distribution factor = 0.75
- Total pavement thickness for CBR 4% and traffic 7.2 msa from IRC:37 2001
chart1 = 660 mm
- Pavement composition can be obtained by interpolation from Pavement
Design Catalogue (IRC:37 2001).
- Bituminous surfacing = 25 mm SDBC + 70 mm DBM
- Road-base = 250 mm WBM
- sub-base = 315 mm granular material of CBR not less than 30 %